The Role of Viscous Particle Segregation in Forming Chromite Layers from Slumped Crystal Slurries: Insights from Analogue Experiments
Bibliographic record
Abstract
The question of how the mineral layering in layered intrusions forms has been extensively debated for many decades. There are many types of layering and it is of course possible that a number of mechanisms are involved. Of particular interest is how chromite layers form, because these may contain valuable metals (such as platinum-group elements) in addition to Cr. One model for the formation of these layers is that they formed through slumping of semi-consolidated cumulates from the margins of the intrusion into the magma chamber. During this slumping, the grains are sorted by density and/or size differences. This study examines the viability of this process using analogue modelling. Starting materials (beads and glycerine) were scaled to match the density and size of the minerals (chromite, orthopyroxene and plagioclase) present in layered mafic-ultramafic layered intrusions and to match the density and viscosity of the silicate magma. A Perspex flume tank divided by a removable partition at one end was fully filled with glycerine. A homogenized mixture of the beads was placed in the smaller partition of the tank (representing the margins of the magma chamber). The tank was then inclined between 16° and 45°. The partition was removed and the beads flowed into the main part of the box. The experiments were recorded by video camera, allowing us to follow the dynamics of the flow during each run. Segregation of the beads was observed in the final deposits: the larger, less dense beads (representing plagioclase) concentrated at the top of the flow, with the intermediate-sized and medium density beads (representing orthopyroxene) in the middle and the smaller, denser beads (representing chromite) at the bottom, thus mimicking natural examples. In experiments where the angle of inclination was low, long, thin layers formed, such as those found in the Bushveld Complex. In experiments where the angle of inclination was high, thick but short layers formed. A dimensionless analysis allows better understanding of the dynamics of the flow. At the macroscopic scale, the flow regime is strongly influenced by the viscosity of the fluid and is considered macro-viscous, where the role of the interstitial liquid is non-negligible.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.002 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.001 |
| Scholarly communication | 0.001 | 0.001 |
| Open science | 0.001 | 0.000 |
| Research integrity | 0.000 | 0.001 |
| Insufficient payload (model declined to judge) | 0.001 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".